Holding arrangement having a device for actively damping vibration

ABSTRACT

In a holding arrangement ( 101 ) for a medical-optical instrument ( 103 ), an electric motor is provided in a rotational joint ( 111, 119 ) to compensate a load torque occurring in this rotational joint. This electric motor is supplied with current in correspondence to a detected position of the rotational joint ( 111, 119 ). A current control curve required for this purpose is stored in a memory. This current control curve can be determined in that the rotational joints are deflected with the electric motor into predetermined positions and the current demand needed therefor is detected. The holding arrangement ( 101 ) has a unit for actively damping vibration including a vibration damping control loop. This vibration damping control loop outputs a superposition motor current to the electric motor as an actuating quantity in order to move the rotational joint ( 111, 119 ) with the electric motor so that a detected vibration of the holding arrangement ( 101 ) is countered.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part application of U.S. patent applicationSer. No. 10/879,037, filed Jun. 30, 2004, and claims priority of Germanpatent application nos. 103 29 549.6, filed Jun. 30, 2003, 10 2004 008381.9, filed Feb. 20, 2004 and 10 2004 063 606.0, filed Dec. 27, 2004,the entire contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to a holding arrangement including a holdingarrangement for a medical-optical instrument. The holding arrangementhas at least one rotational joint and has an apparatus for balancing aload torque which is caused by the medical-optical instrument on therotational joint. The apparatus for compensating load torque includes anelectric motor.

BACKGROUND OF THE INVENTION

A holding arrangement of the above type is disclosed in U.S. Pat. No.6,963,444. There, a stand arrangement for a medical-optical instrumentis described. In this stand arrangement, the medical-optical equipmentis held on a front arm which is coupled via a rack and pinion gearassembly to an electric motor. The holding arrangement includes avibration sensor with a control loop. This control loop makes itpossible to control the electric motor so that vibrations of themedical-optical equipment on the front arm are actively countered.

An adjustable stand for a surgical microscope is described in U.S. Pat.No. 5,492,296. The adjustable stand includes first and second rotationaljoints. An elastic energy store is assigned to each one of theserotational joints. The elastic energy store includes a torsion springhaving a pretension which can be adjusted. The elastic energy storesgenerate a compensating torque which counters a load torque in therotational joints caused by the surgical microscope accommodated on thestand.

U.S. Pat. No. 5,667,186 discloses a holding arrangement for amedical-optical instrument wherein motorically-adjustable balancingweights are provided in order to compensate load torques occurring atthe rotational axes of the holding arrangement.

U.S. Pat. No. 5,642,220 discloses a holding arrangement for amedical-optical instrument wherein a linear spring unit or a gaspressure cylinder is provided for generating a counter torque tocompensate load torques. The linear spring unit or the gas pressurecylinder operate on a lever arm. A desired compensating torque can beadjusted in that a point of application of the gas pressure cylinder orlinear spring unit is varied.

U.S. Pat. No. 5,402,582 discloses a holding arrangement foraccommodating a probe head for measuring workpieces. The holdingarrangement includes a multi-joint carrier arm. Torsion springs areprovided in the joints of the carrier arm. These torsion springsgenerate torques which counter the load torques in these joints.

U.S. Pat. No. 5,332,181 discloses a motorized stand having a surgicalmicroscope as a holding arrangement for a medical-optical instrument.This stand has a support column which is supported on a stand base andcan be rotated about a vertical axis. A multi-joint carrier arm isarranged on this carrier column and has four rotational joints withmotorized drives. A control unit is assigned to these motorized drives.The control unit is connected to angular transducers which are arrangedon the rotational joints. The desired position of a specific rotationaljoint is inputted to the control unit. The drives of the holdingarrangement are then supplied with current in correspondence to thepregiven joint position of a rotational joint in order to move aspecific carrier arm section on a rotational joint into a desiredangular position.

U.S. Pat. No. 6,471,165 discloses a surgical microscope having a standwhich has a motorically-adjustable pivot axis running essentiallyhorizontally. A step motor is disposed in this pivot axis. The stepmotor is controlled by an operator-controlled element and a servoadjustment of the surgical microscope, which is accommodated on thisaxis, is made possible. Force sensors or torque sensors are provided inthe operator-controlled element.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a holding arrangementwherein an equilibrium state can be adjusted for the force-free movementof an instrument about a rotational joint with the instrument beingaccommodated on the holding arrangement. It is a further object toprovide a servo-controlled movement of the instrument about thisrotational axis without disturbing vibrations of the instrumentoccurring at the holding arrangement.

According to a feature of the invention, the apparatus for compensatingthe load torque includes an electric motor which is combined with adetecting unit for detecting the position of the rotational joint andcan be supplied with current in dependence upon a detected rotationaljoint position for generating a counter torque balancing the loadtorque. For this purpose, a control unit is provided which adjusts therequired motor current. To compensate the load torque, the control unitassigns a value for the motor current to a detected rotational jointposition value and this motor current is outputted to the electric motorand causes the electric motor to generate a counter torque whichcompensates the load torque applied to the rotational joint. A unit foractively damping vibration is provided in the holding arrangement. Thisunit for actively damping vibration includes a sensor for detectingvibrations of the holding arrangement. The sensor provides a controlquantity for a vibration-damping control loop. This vibration-dampingcontrol loop outputs an actuating variable in the form of asuperposition motor current to the electric motor in order to move therotational joint so that a detected oscillation or vibration of theholding arrangement is countered. In this way, a compactly configuredholding arrangement is provided which can be motorically tilted andpivoted and is easily adaptable to different configurations of amedical-optical instrument for torque compensation.

According to another embodiment of the invention, the sensor fordetecting vibrations of the holding arrangement is configured as anacceleration sensor. Vibrations of medical-optical instruments on theholding arrangement can be detected in that, for example, the sensor ismounted directly on this instrument. If the sensor is configured as aflexure or bending sensor, then the bending sensor can be assigned to acarrier arm of the stand arrangement in order to draw a conclusion as tooscillations or vibrations of the arrangement from a time-dependentchange of the bending of the carrier arm. It is also possible toconfigure the sensor to detect vibrations of the holding arrangement asa motion sensor. The motion sensor is mounted on component assemblies ofthe holding arrangement which move because of oscillations orvibrations.

In a further embodiment of the invention, a brake is assigned to therotational joint. In this way, it can be ensured that the holdingarrangement does not move when no current is supplied to the electricmotor.

In a further embodiment of the invention, the electric motor is coupledto the rotational joint by means of a gear assembly. In this way, aprecise adjustment of an equilibrium state is made possible in theholding arrangement.

In a further embodiment of the invention, the electric motor includes adrive axis which extends offset to a rotational axis of the rotationaljoint. In this way, space for connecting apparatus to themedical-optical instrument is provided in the holding arrangement and,it is, for example, possible to undertake an optical beam decoupling atthe particular rotational axis.

In a further embodiment of the invention, the means for detecting theposition of the rotational joint include an encoder of the electricmotor or a position transducer. In this way, an instantaneous positionof the rotational joint can be precisely determined.

In another embodiment of the invention, an electronic memory is assignedto the control unit of the electric motor wherein a curve is stored ofthe current as a function of the rotational joint position or a tablehaving current values and rotational joint positions corresponding toeach other. In this way, a rapid assignment of the required currentvalue for a given position of the medical-optical equipment is ensured.

In another embodiment of the invention, at least two rotational jointsare provided with means for compensating a load torque. In this way, itis made possible that a medical-optical device, which is accommodated onthe holding arrangement, can be moved force-free in correspondence toseveral degrees of freedom of movement.

In a further embodiment of the invention, means for detecting atime-dependent change of the position of the rotational joint areprovided in the holding arrangement. These means preferably detect atime-dependent change of the rotational joint position via amathematical derivative of the determined rotational joint position as afunction of time. The determined change of the rotational joint positionis supplied as a control variable to a control loop which outputs amotor current for the electric motor on the rotational joint as apositioning quantity. This motor current is superposed on the motorcurrent for torque compensation so that the motor generates anadditional torque which counters a determined change of the rotationaljoint position.

With such a control loop, it is possible to simulate an inertial effectto an operator. Accordingly, for example, for a holding arrangementconfigured as a manipulator, it can be avoided that the trembling of ahuman hand is superposed on the instrument itself with the human handguiding the instrument accommodated on the holding arrangement. At thesame time, such a control loop makes possible that non-predefinableforces and torques are detected as a real touch feedback withoutfalsifying external forces. These predefinable forces and torques are,in surgery, for example, cutting and return forces when cutting elastictissue and when performing a resection or, other than surgery, whenpicking up an unknown item by the operator with a corresponding tool.

In the area of medicine, physicians, for example, are hereby placed inthe position to keep their hands clear of a surgical area. This affordsthe possibility to utilize radiation-intensive intraoperative imagingmethods during surgery and to be able to also treat highly infectiouspatients. Low vibration and precise movements can be carried out with aholding arrangement configured as a manipulator. For this reason, withthe use of such a manipulator with a surgical microscope, a preparationintensive navigation is, as a rule, no longer necessary for a navigationutilized often for precise interventions.

With a corresponding active superpositioning of current curves orcurrent control curves of several electric motors of the manipulator inthe weight-equalized state, semi-robotic functions can be realized asrequired. For example, the user, with a suitable control, can be keptaway from critical regions of the surgical area either entirely or hecan be warned by an artificial resistance as long as the user wantsthis. For this purpose, the data of navigation tools, virtual 3D modelsor 3D tracks in the corresponding motor positions can be converted toadditively superposed motor currents. In the area of surgery, it can beespecially ensured that surgery takes place only in the peripheralregion of a tumor.

Generally, the described control principle (open loop and/or closedloop) for a holding arrangement has the advantage compared to theclassic robot technology that it needs no force sensors and/or no torquesensors and no complex sensor actuating control need be used, which isdifficult to manage and with specific dynamic regions which are onlyaccessible with difficulty.

If, in the holding arrangement, the mass distribution of the carrierarms is so selected that at least approximately a weight compensationabout the rotational axis is provided for the particular joint, thencomparatively weak motors can be used for shifting the holdingarrangement. These motors must then only compensate slight torques. In aholding arrangement, whose carrier arms are balanced about therotational axes of rotational joints, it would, for example, only benecessary that the motors compensate the torques caused in therotational axis by a tool which is taken up in addition.

In a further embodiment of the invention, the medical-optical instrumentis accommodated with a parallelogram arm on a carrier arm. Such aparallelogram arm makes it possible that the means for compensating aload torque can be ergonomically favorably mounted in the region of astand arm above the medical-optical instrument. Furthermore, a stableaccommodation of the medical-optical instrument on the holdingarrangement is ensured in this way.

In a method for determining a current control curve for adjusting anequilibrium state in a holding arrangement of the invention, thefollowing takes place: the at least one rotational joint is moved bymeans of the electric motor about an axis of the rotational joint; thecurrent demand of the electric motor for moving the rotational joint isdetermined; the instantaneous position of the rotational joint isdetermined; and, the determined current demand in dependence upon therotational joint position is stored in an electronic memory as a currentcontrol curve. In this way, an equilibrium state can be adjusted for theholding arrangement for different configurations of medical-opticalequipment.

It is also possible to determine a current control curve in that the atleast one rotational joint is moved by means of the electric motor in afirst direction. The current demand of the electric motor, which isneeded for moving the rotational joint, is determined in dependence uponthe position of the rotational joint and thereafter, the at least onerotational joint is moved by means of the electric motor in a seconddirection opposite to the first direction. The current demand of theelectric motor, which is needed for the movement of the rotationaljoint, is determined in dependence upon the position of the rotationaljoint.

Preferably, a mean value of the current demand, which is needed for themovement of the at least one rotational joint in the first direction,and of the current demand, which is needed for the movement of the atleast one rotational joint in the second direction, is computed and isstored in dependence upon the rotational joint position in an electronicmemory as a current control curve. In this way, it is possible togenerate a current control curve which is not burdened by errors whichare caused by the friction forces in the particular rotational joint.

For the determination of the current control curve of the at least onerotational joint, it is sufficient to move the rotational joint with theelectric motor over a rotational angular section Δφ, for example, |Δφ|or |Δφ|≦π/2 or |Δφ|≦π/4 because a conclusion can be drawn from a sectionof the detected current control curve as to the total course of thecurrent control curve in the angular range 0≦φ≦2π which corresponds to acomplete revolution of the rotational joint. In this way, it is possibleto take up a desired current curve or current control curve for therotational joint within a short time, if needed, within a few seconds.

For the holding arrangement, an instantaneous position of the rotationaljoint is detected and the electric motor is supplied with currentcorresponding to a current control curve stored in a memory. In thisway, an equilibrium state can be established for the medical-opticalequipment in that a current value for torque compensation for theelectric motor is assigned to a specific position of the rotationaljoint.

It is further also possible to determine an instantaneous change of theposition of the rotational joint and to then output a current to theelectric motor which counters the change of the position of therotational joint.

If several rotational joints are provided in the holding arrangement,which rotational joints have apparatus for compensating a load torquewith electric motors, an equilibrium state can be adjusted in that aninstantaneous position of a first rotational joint is determined, aninstantaneous position of a second rotational joint is determined and anelectric motor, which is assigned to the first rotational joint, and anelectric motor, which is assigned to the second rotational joint, issupplied with current corresponding to a two-dimensional current controlcurve stored in a memory. The current control curve assigns acorresponding current value for the torque compensation to therotational joints corresponding to the specific instantaneous positionof the rotational joints.

In order to determine a two-dimensional current control curve for theadjustment of an equilibrium state in a holding arrangement, theposition of a first rotational joint is detected and, for a knownposition of the first rotational joint, a second rotational joint ismoved about its axis by means of an electric motor assigned to thesecond rotational joint and then, the current requirement of theelectric motor is determined which is needed for moving the secondrotational joint. Thereafter, the instantaneous position of the secondrotational joint is detected and the specific current requirement isstored in dependence upon the position of the second rotational joint inan electronic memory as a first current control curve. Thereafter, in aknown position of the second rotational joint, the first rotationaljoint is moved about its axis by means of the assigned electric motor.The current demand of the electric motor, which is needed for themovement, is determined, the instantaneous position of the firstrotational joint is determined and then, the determined current demandis stored as a second current control curve in an electronic memory independence upon the position of the second rotational joint.

Corresponding methods can be applied for adjusting the equilibrium statein a holding arrangement with three or more rotational joints in thatsuitable three-dimensional or multi-dimensional current control curvesare determined for electric motors, which are assigned to the rotationaljoints, and are applied for driving the electric motors.

In a holding arrangement, which is configured as a manipulator, it mustbe ensured that, for each newly picked up instrument, tool or workpiece,either first a calibration of position-dependent motor currents is madeor, for each taken-up item, corresponding identifications together withthe absolute or additive position-dependent compensation motor currentcurves can be called up, for example, from an electronic memory. Forthis purpose, items, which are intended to be taken up by the holdingarrangement, should be provided with an automatic identification via abarcode or a microchip. Furthermore, it is possible to apply the methodsfor tool identification, which are known from the manufacturingindustry, such as for automatic supply devices for tool machines.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the drawingswherein:

FIG. 1 shows a holding arrangement for a surgical microscope in a firstposition;

FIG. 2 shows the holding arrangement of FIG. 1 in a second position;

FIG. 3 is a schematic of a first rotational joint of the holdingarrangement of FIG. 1;

FIG. 4 is a schematic of a second rotational joint of the holdingarrangement of FIG. 1;

FIG. 5 shows how a rotational torque occurs in a rotational joint of theholding arrangement of FIG. 1;

FIG. 6 shows the interrelationship between rotational joint position anda torque occurring at the particular rotational joint;

FIG. 7 shows a circuit arrangement for controlling an electric motor ina rotational joint of the holding arrangement of FIG. 1;

FIG. 8 is a schematic of a rotational joint having a medical-opticalinstrument and an electric motor;

FIG. 9 shows motor current curves of the electric motor in therotational joint of FIG. 8;

FIG. 10 shows a holding arrangement with a surgical microscope;

FIG. 11 is a schematic of a circuit arrangement for controlling severalelectric motors in a circuit arrangement having several rotationaljoints;

FIG. 12 is a schematic of a holding arrangement configured as amanipulator;

FIG. 13 is a schematic of a circuit arrangement with a control loop forcontrolling an electric motor in a rotational joint of the holdingarrangement of FIG. 9 configured as a manipulator; and,

FIG. 14 is a circuit arrangement with control loops for controllingelectric motors in several rotational joints of a holding arrangement.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 shows a holding arrangement 101 having an articulatedparallelogram arm 102 on which a medical-optical unit in the form of asurgical microscope 103 is accommodated. The holding arrangement 101 isattached to a stand (not shown) by means of a support guide 104. Theholding arrangement 101 can be rotated about a vertical rotation axis105 on this support guide 104.

The articulated parallelogram 102 includes parallelogram arms 106 to 110having rotational joints 111 to 117. A first electric motor is assignedto the rotational joint 111. This electric motor makes possible acontrolled movement of the articulated parallelogram 102 about ahorizontal rotational axis 118. The surgical microscope is pivotedlaterally with this movement.

A further rotational joint 119 having a rotational axis 120 isaccommodated on the arm 110. An electric motor is also assigned torotational joint 119. With this electric motor, a tilt movement of thesurgical microscope 103 can be controlled about the rotational axis 120.

An acceleration sensor 121 is attached to the surgical microscope 103 asa sensor for detecting oscillations in order to detect vibrations of themedical-optical instrument which is accommodated on the holdingarrangement 101.

FIG. 2 shows the holding arrangement 101 of FIG. 1 in a deflectedparallelogram position. The units of the holding arrangement areidentified by the same reference numerals used in FIG. 1. The surgicalmicroscope 103 in FIG. 2 is pivoted laterally relative to the positionof the surgical microscope in FIG. 1.

FIG. 3 schematically shows the rotational joint 111 of holdingarrangement 101 of FIG. 1. The rotational joint 111 has a first jointpart 301 and a second joint part 302 which can be moved relative tojoint part 301. The joint part 302 is journalled on the joint part 301with support units 303 and 304. An electric motor 305 is assigned to therotational joint 111 which is connected to the joint part 302 by meansof a shaft 306. A torque can be generated with the electric motor 305which is introduced into the second joint part 302 of the rotationaljoint 111.

The electric motor 305 has an encoder 307. This encoder 307 makes avoltage signal available from which, with a suitable signal processingunit, an instantaneous position of the electric motor and therefore ofthe shaft 306 can be derived. The position of the rotational joint 111can therefore be determined from the voltage signal of the encoder 307.

In the rotational joint 111, a magnetic brake 308 is provided which,dependent upon the drive, enables or disables a movement of the secondjoint part relative to the first joint part.

With the torque, which the electric motor 305 provides, either a loadtorque, which is applied to the second joint part 302, can becompensated or the joint part 302 can be moved in correspondence withthe drive of the electric motor 305.

FIG. 4 schematically shows the further rotational joint 119 of FIG. 1.As with rotational joint 111, an electric motor 405 is also assigned tothe rotational joint 119 with which a torque, which is applied to thejoint part 402, can be compensated. The electric motor 405 is held in afirst joint part 401 of the rotational joint 119. The rotational joint119 further includes bearing units 403 and 404 which make possible amovement of the second joint part 402 relative to the first joint part401.

The electric motor 405 is coupled via a gear assembly 406 to the secondjoint part 402. This gear assembly includes a drive pinion 407 which isarranged on the drive shaft of the electric motor 405. This drive pinion407 meshes with a toothed gear 408 which is fixedly connected to thesecond joint part.

In order to be able to enable or disable a movement of first joint part401 and second joint part 402 even for an electric motor withoutcurrent, a magnetic brake 409 is provided in the rotational joint 119.

The rotational joint 119 further includes a position transducer 410which makes available a voltage signal which corresponds to aninstantaneous position of the toothed gear 408 on the second joint part402 of the rotational joint 119.

It will now be explained with respect to FIGS. 5 to 7 how a load torque,which occurs at the rotational joints 111 or 119 of FIG. 1, andoscillations or vibrations can be compensated with the electric motor inthe rotational joints.

For this purpose, FIG. 5 shows schematically a torque 501 which occurson a rotational joint 502 because of a load having the centroid 503which is taken up at the rotational joint 502 with the lever arm 504because this load is subjected to a weight force 505. As a function ofthe angle φ between the weight force 505 and the lever arm 504, thereresults a dependency of the magnitude of the torque {right arrow over(D)} occurring at rotational joint 502 which is shown in FIG. 6.

The following equation applies: {right arrow over (D)}=LMg sin φwherein:

-   -   L is the length of the resulting lever arm;    -   M is the mass of the centroid;    -   g is the acceleration of gravity constant; and,    -   φ is the angle between the lever arm and the direction of the        weight force.

An equilibrium state is adjusted at the rotational joints 111 and 119 inthat the electric motor in the rotational joints of the holdingarrangement 101 of FIG. 1 is so supplied with current that itcompensates a load torque occurring at the rotational joints.

For automatically adjusting such an equilibrium state, the electricmotors in these rotational joints are wired in correspondence to acircuit arrangement shown in FIG. 7. The circuit arrangement 701includes a motor control unit 702 which is connected to the electricmotor 703. Signals from a position transducer 704 are supplied to themotor control unit 702. The position transducer 704 is configured as anangle sensor or as an encoder. This position transducer 704 outputs aninstantaneous angle position of the rotational joint. Corresponding toan instantaneously detected angle position of the rotational joint, acurrent control curve is read out which is stored in an electronicmemory 705. This current control curve corresponds to the current value,which is required in each position of the rotational joint, for torquecompensation by the electric motor.

Accordingly, if the position of the surgical microscope 103 of FIG. 1 isso changed that the rotational joints 111 or 119 are moved by theelectric motors, then the corresponding motor control unit controls themotor current in correspondence to the instantaneous rotational jointpositions in such a manner that a torque balance occurs in therotational joints. For this purpose, a current control curve, which isstored in the particular electronic memory, is read out for eachrotational joint (111, 119). This current control curve is dependentupon the positions of the two rotational joints (111, 119) and on themass distribution of the medical-optical instrument taken up at thecorresponding holding arrangement. If the mass distribution is changed,for example, in that peripheral apparatus is connected to themedical-optical instrument, then a modified current control curve mustbe accessed for the torque compensation in the rotational joints.

Such a current control curve can basically be determined in a simplemanner. For this purpose, the current, which is needed for moving theholding arrangement about the particular rotational joints by means ofelectric motors, is detected as a function of the instantaneouspositions of these rotational joints and is stored in the particularelectronic memory. For example, the rotational joint 111 is moved into aknown position and, thereafter, the current control curve for therotational joint 119 is recorded. In a next step, the rotational joint119 is moved into a known position and the corresponding current controlcurve for the rotational joint 111 is determined. From the current curvedetermined in this manner, a two-dimensional set of current data forcompensation in each position of the rotational joints 111 and 119 canbe determined by means of trigonometric functions.

Servo switches 706 are assigned to the motor control unit 702 in orderto make possible a servo operation of the electric motor 703 for arotational joint of the described holding arrangement. Such a servooperation can, for example, be of advantage for a fine adjustment of themedical-optical instrument on the holding arrangement.

The circuit arrangement 701 includes a vibration damping control loop707 to which a sensor is assigned for detecting the oscillations orvibrations of the equipment accommodated by the holding arrangement. Thevibration damping control loop 707 outputs a current signal to theelectric motor 703 which is superposed upon that from the motor controlunit 702. The sensor 708 for detecting the oscillations or vibrationssupplies a signal which serves a control quantity for the vibrationdamping control loop 707. The control quantity is an index for themagnitude of an oscillation or vibration of the equipment accommodatedby the holding arrangement. From this control quantity, the vibrationdamping control loop 707 forms the current signal for the electric motor703 as an actuating variable in such a manner that the detectedoscillation or vibration is countered by the movement of the electricmotor 704.

In FIGS. 8 and 9, a further method is described for determining acurrent control curve for torque compensation at a rotational joint of aholding arrangement.

FIG. 8 shows a rotational joint 800 of a holding arrangement whichsupports a mass, which is rotatably journalled about an axis 801, in theform of a medical-optical instrument 802. The medical-optical instrument802 is subjected to a gravity force in the direction of arrow 803. Thisgravity force causes a load torque 804 in the axis 801 of the rotationaljoint 800. To compensate this load torque 804, a drive unit havingelectric motor 805 is assigned to the rotational joint 800. The electricmotor 805 is coupled to the axis 801 of the rotational joint by means ofa gear assembly 806 and can so move the medical-optical instrument 802in the direction of arrows (807, 808). For a movement of themedical-optical instrument 802, friction forces and acceleration forcesin general occur at the rotational joint 800. These forces areespecially dependent upon the direction in which the medical-opticalinstrument 802 is displaced at the rotational joint 800.

FIG. 9 shows, in a graph 903, a first motor current curve 901 for amotor current for the electric motor 805 of FIG. 8 as a function of theangular position p of the rotational joint 800 of FIG. 8 in order tomove the medical-optical instrument 802 in the direction of the arrow807. A motor current curve 902 corresponds to the motor current of theelectric motor 805 of FIG. 8 which is necessary to move themedical-optical instrument in the direction of arrow 803 of FIG. 8.

The motor current curves 901 and 902 have noise caused by measuringoperations and are displaced parallel to the abscissa of the graph 903.A non-noisy motor current curve 904 results from the formation of a meanvalue of the motor current curves 901 and 902 by means of suitablemathematical averaging algorithms. This motor current curve 904corresponds to a torque at rotational joint 800, which can be generatedby means of electric motor 805 of FIG. 8 and which, for a givenrotational joint position, makes possible an exact static torquecompensation. This motor current curve is neither made erroneous byfriction forces nor by acceleration forces because the contribution ofthese forces are eliminated by the formation of the corresponding meanvalue.

In order to compute a suitable motor current curve for torque balancing,it is not necessary to move the medical-optical instrument 802 on therotational joint 890 of FIG. 8 in the angular range 0≦φ≦2π about theaxis of the rotational joint 801.

Since it is known that the static load torque in the rotational jointsatisfies the relationship explained with respect to FIG. 6, it ispossible by means of suitable mathematical algorithms to draw aconclusion as to a motor current curve in the angular range 0<φ<2π fromthe detected course of the motor current curves in an angular range 905or 906 in FIG. 9.

FIG. 10 shows a surgical microscope 1001 taken up on a holdingarrangement 1000. The surgical microscope 1001 is rotatably journalledon the holding arrangement 1000 about an axis 1002 and can be displacedin the angular range indicated by the arrow 1003. To balance loadtorques at desired angular positions of the surgical microscope 1001, anelectric motor 1004 is provided which operates on the surgicalmicroscope 1001 by means of a gear assembly.

To record a suitable motor current curve for torque compensation, it ishere sufficient to move the surgical microscope 1001, for example, overone of the angular ranges indicated by the arrows 1005, 1006 or 1007. Inthis way, and even when a movement of the surgical microscope 1001 aboutthe axis 1002 is restricted because of connected ancillary apparatus, anappropriate motor current curve for torque compensation can bedetermined over the entire accessible angular range.

FIG. 11 is a schematic of a circuit arrangement for controlling severalelectric motors in a circuit arrangement having several rotationaljoints. The circuit arrangement 1101 has a motor control unit 1102 whichis connected to electric motors 1103 ₁, 1103 ₂, . . . 1103 _(n). Theseelectric motors 1103 ₁, 1103 ₂ . . . 1103 _(n) are assigned torotational joints 1104 ₁, 1104 ₂ . . . 1104 _(n). Each of the rotationaljoints having an electric motor includes a position transducer orencoder with which the instantaneous angular position of the rotationaljoint can be determined. Further, in the circuit arrangement 1101,vibration damping control loops 1105 ₁, 1105 ₂, . . . 1105 _(n) areprovided. These vibration damping control loops 1105 ₁, 1105 ₂, . . .1105 _(n) are supplied with the signal of an oscillation or vibrationsensor 1106. It should be noted that basically, in the circuitarrangement, also several such oscillation or vibration sensors can beprovided which are mounted at various locations in the holdingarrangement.

A multi-dimensional current control curve can be determined in that,first, the respective angular positions of all rotational joints 1104 ₁,1104 ₂ . . . 1104 _(n) are determined. The multi-dimensional currentcontrol curve can be applied as the basis for adjusting an equilibriumin a holding arrangement having multiple rotational joints. In the knownposition of the rotational joints 1104 ₂, . . . 1104 _(n), a currentcontrol curve as shown in FIG. 7 is recorded for the electric motor 1103₁ of the rotational joint 1102 ₁ and is stored as an n-dimensional dataset in an electronic memory 1105. Thereafter, a corresponding currentcurve is recorded for the rotational joint 1103 ₂ at known positions ofthe remaining rotational joints, et cetera.

After the determination of a current curve set, a current data set canbe computed via conversion with corresponding trigonometric functionsfor all rotational joints 1104 ₁, 1104 ₂, . . . 1104 _(n) at knownangular positions of all rotational joints. The current data setprovides a current for equilibrium for each electric motor 1103 ₁, 1103₂ . . . 1103 _(n). With the aid of the vibration damping control loops1105 ₁, 1105 ₂, . . . 1105 _(n), the circuit arrangement can be sodriven that mechanical vibrations from a load are countered which loadis accommodated on a corresponding holding arrangement.

FIG. 12 shows a holding arrangement configured as a manipulator 1200with the holding arrangement having several rotational joints. Theholding arrangement includes an arm 1201 which is mounted on a stand1202 and can there be moved with rotational axes 1203 and 1204. Electricmotors 1205 and 1206 are assigned to rotational axes 1203 and 1204,respectively. The arm 1201 is connected via a rotational joint 1207 toarm 1208. The arm 1208, in turn, holds an arm 1210 via a rotationaljoint 1209. On this arm 1210, an instrument receptacle unit 1212 isdisposed on a rotational joint 1211 and has a unit for accommodating atool in the form of an instrument holder 1213. The instrument holder1213 holds a medical instrument in the form of a surgical tool 1220.

A handle 1214 is provided on the instrument receptacle unit 1212. Anoperator can control the manipulator 1200 with the handle 1214. Electricmotors 1215, 1216 and 1217 are mounted on rotational joints 1207, 1209and 1211, respectively. The stand 1202 is, in turn, disposed on a standconsole 1218 and can there be rotated about a vertical axis 1219. Withthe handle 1214, an operator can move the instrument 1220, which isaccommodated on the instrument holder 1213, in the directions indicatedby arrows 1221, 1222, 1223 and 1224. A bending sensor 1230 is providedon the arm 1208 of the manipulator 1200 for detecting oscillations.Optionally, bending sensors can also be provided on the remaining arms1201 and 1210.

Angle transducers 1225, 1226, 1227 and 1228 are provided at therespective rotational joints of the manipulator 1200. The instantaneousposition of the respective rotational joints can be detected utilizingthe respective angle transducers 1225, 1226, 1227 and 1228. The signalsof the angle transducers 1225, 1226, 1227 and 1228 are supplied to acontrol unit 1229 which controls the electric motors 1215, 1216 and 1217for torque balancing in correspondence to the manner explained withrespect to FIG. 11. This permits an operator to guide the manipulator1200 force-free via the handle 1214 in correspondence to the directionsindicated by the arrows 1221, 1222, 1223 and 1224. The signals from oneor several bending sensors are supplied to corresponding control loopswhich output current signals to the electric motors in order to activelysuppress detected oscillations or vibrations.

FIG. 13 is a schematic of a circuit arrangement 1301 for controlling theelectric motor 1203 in a rotational joint of the holding arrangement ofFIG. 12 configured as a manipulator. The circuit arrangement 1301 ismodified compared to FIG. 7 or FIG. 11.

The circuit arrangement 1301 includes a motor control unit 1302 which isconnected to the electric motor 1303. The circuit arrangement 1301includes a position transducer 1304 which supplies data as to theinstantaneous position of the rotational joint having the electric motor1303 to the control unit 1302. In correspondence to the circuitarrangement 701 of FIG. 7, the circuit arrangement 1301 includes anelectronic memory 1305 wherein a current control curve is stored. Thecurrent control curve contains the data of a current for the electricmotor on the particular rotational joint as a function of the rotationaljoint position. This current for the electric motor is needed for torquebalancing. The current control curve can be stored, for example, as amathematical function or as a value table. Here, it can be provided tointerpolate, as needed, intermediate values by means of a suitablemathematical function. For an instantaneous rotational joint position,the motor control unit 1302 generates a motor control signal ME fortorque compensation in the rotational joint having electric motor 1303.

As a difference with respect to the circuit arrangement 701 of FIG. 7,the circuit arrangement 1301 is additionally provided with a unit fordetecting the time-dependent change of the rotational joint position1306. The unit for detecting the time-dependent change of the rotationaljoint position 1306 is connected to the position transducer 1304. Theunit determines the time-dependent change of the rotational jointposition via a time-dependent derivative of the data as to therotational joint position which is supplied from the position transducer1304. As an alternative, it is, for example, also possible to detect thetime-dependent change of the rotational joint position by evaluating themotor current in the electric motor 1303 of the corresponding rotationaljoint.

The data of the time-dependent change of the rotational joint position1306 is likewise supplied to the motor control unit 1302. There, aclosed control loop 1307 stores the detected time-dependent change ofthe rotational joint position as a control quantity. The control loop1307 outputs a motor control signal M_(R) as an actuating quantity. Thecontrol loop 1307 is configured as a PID control loop to which, as a setvalue, the value {dot over (φ)}_(des)=0 is pregiven as a value for awanted time-dependent change of the rotational joint position. It is,however, noted that the control loop can also be configured inaccordance with another control principle known to those skilled in theart.

Because of the selected desired value {dot over (φ)}_(des)=0, the motorcontrol signal M_(R) corresponds to a motor current in the electricmotor 1303 which counters a displacement of the rotational joint.

In the motor control unit 1302, the motor control signal M_(R), which isoutputted by the control loop 1307, is superposed on the motor controlsignal M_(E) for torque balancing in the rotational joint havingelectric motor 1303 at a given angular position.

An operator, who shifts the corresponding rotational joint, for example,with a handle 1214 shown in FIG. 12, perceives this counter action ofthe electric motor as a shift resistance corresponding to an inertialforce. The shift resistance is dependent upon a displacement speed.

The dependency of the shift resistance on a shift speed can be adjustedto a desired value by selecting the time constant in the PID controlloop.

By utilizing corresponding control loops, it is basically also possibleto assign a desired shift resistance to a given shift speed.

The circuit arrangement 1301 makes possible to move an instrument, whichis accommodated on the holding arrangement, in equilibrium about arotational joint, for example, with the handle 1214 of FIG. 12 withoutthe need for torques to be generated by the operator which would counterthe torques which occur in the particular rotational joint because of adisplacement of the mass center of gravity of the accommodatedinstrument. At the same time, with the movement of the handle, theoperator perceives a touch-perceptible resistance which, for example,prevents that tremors of a human hand are transmitted to the instrumentaccommodated on the holding arrangement. Non-predefinable forces andtorques (for example, cutting forces and return forces when cuttingtissue) must be developed for the manipulator by the operator. This is,however, desirable because, in this way, the operator has a real,touch-perceptible feedback without falsifying external forces.

FIG. 14 shows a schematic of a further circuit arrangement 1401 forcontrolling several electric motors 14031, 1403 ₂ . . . . 1403 _(n)which are arranged in corresponding rotational joints of a holdingarrangement configured as a manipulator as explained basically withrespect to FIG. 11. The circuit arrangement includes also a vibrationdamping control loop and, optionally, also several vibration dampingcontrol loops which are, however, not shown. The circuit arrangement1401 has a motor control unit 1402 which is connected to the electricmotors 1403 ₁, 1403 ₂ . . . 1403 _(n) at the corresponding rotationaljoints whose rotational joint positions are detected by positiontransducers 1404 ₁, 1404 ₂, . . . 1404 _(n). In correspondence to thecircuit arrangement 1101 of FIG. 11, the circuit arrangement 1401includes an electronic memory 1405 wherein a multi-dimensional currentcontrol curve is stored which contains the data of a motor current forequilibrium M_(E1), M_(E2), . . . M_(En) for electric motors 1403 ₁,1403 ₂ . . . 1403 _(n) for a given position of the rotational joints.

In contrast to the circuit arrangement 1101, units for detecting thetime-dependent change of the position of the particular rotationaljoints 1406 ₁, 1406 ₂, . . . 1406 _(n) are provided in the circuitarrangement 1401. The units supply the data of a time-dependent changeof the rotational joint position to the motor control unit 1402.

In the motor control circuit 1402, this information is supplied as acontrol quantity to the control loops 1407 ₁, 1407 ₂, . . . 1407 _(n).These control loops output motor control signals M_(R1), M_(R2) . . .M_(Rn) as an actuating quantity. Corresponding to the circuitarrangement explained with respect to FIG. 13, each of the motor controlsignals M_(R1), M_(R2) . . . M_(Rn) counters the shift of the rotationaljoints to which the corresponding electric motors 1403 ₁, 1403 ₂ . . .1403 _(n) are assigned.

In the motor control unit 1402, the motor control signals M_(R1), M_(R2). . . M_(Rn), which are outputted by the control loops 1407 ₁, 1407 ₂ .. . 1407 _(n), are superposed on the motor control signals M_(E1),M_(E2), . . . M_(En) for torque balancing by electric motors 1403 ₁,1403 ₂ . . . 1403 _(n).

With a manipulator as holding arrangement having several rotationaljoints, the circuit arrangement thereby makes it possible to guide aninstrument via a suitable handle in equilibrium, that is, to move thesame apparently force-free for an operator without, for example, thetremors of the human hand being transferred to the instrument. Therotational joints are driven by corresponding electric motors.

A holding arrangement, which is configured as a manipulator and which iscontrolled via a circuit arrangement shown in FIG. 11, thereby permits aprecise tremor-free guidance of microsurgical instruments, especiallyinjectors, endoscopes or laparoscopes. Basically, such a manipulator canbe provided with an electronic drive in each axis of movement. The drivecan be controlled (open loop and/or closed loop) in a suitable manner.With such a manipulator, implants, permanent pharmaceuticals, sensors,actuators or even detectors and the like can be precisely positioned ona patient.

Such a manipulator can also carry a probe head for measuring workpiecesor a gripping work tool. It is basically also possible, with acorresponding manipulator, to pick up heavy instruments, items or toolswhich can then be fine-motorically moved by an operator. For example,especially, heavy items can be precisely positioned, fixed or assembled.

If an arm configuration is selected for the manipulator (which takesinto account a weight compensation about corresponding rotational jointsvia a suitable mass distribution), then it is possible to utilizecomparatively weak electric motors for adjusting a torque compensationin the axes of movement. This can also make possible a manual operationof the manipulator without support of electric motors. Especially, onlythe comparatively low torques of a weak electric motor have to beovercome.

For working with the manipulator, it can be provided to identify anitem, which is to be picked up by the manipulator, via a barcode or bytriggering a microchip and, in correspondence to a known massdistribution of the picked-up item, to then set suitable motor currentcontrol curves for torque compensation in the memory of a control unitassigned to the manipulator.

For the sake of completeness, it is noted that a corresponding workpieceor tool can be identified as an item, which is picked up by themanipulator, also by the identification principle of automatic feeddevices in machine tools in the forms of magazines or changers.

Compared to classic robot technology, the holding arrangement describedaffords the advantage that it does not require costly force-torquesensors. In this robot technology, complex sensor actuator controls forservo operation of robotic arms driven by motors must be used.

It is understood that the foregoing description is that of the preferredembodiments of the invention and that various changes and modificationsmay be made thereto without departing from the spirit and scope of theinvention as defined in the appended claims.

1. A holding arrangement for an instrument including a medical-opticalinstrument, the holding arrangement comprising: at least one rotationaljoint which is subjected to a load torque by said instrument; and, anapparatus for balancing said load torque applied to said rotationaljoint; said apparatus including: an electric motor; a detecting unit fordetecting a position of said rotational joint and for providing a signalrepresenting a position value for said position of said rotationaljoint; and, said electric motor being combined with said detecting unit;a control unit for receiving said signal and assigning a motor currentto said position value; said control unit outputting said motor currentto said electric motor so as to cause said electric motor to generate acounter torque to balance said load torque applied to said rotationaljoint; a device for actively damping vibrations and said deviceincluding: a sensor for detecting a vibration of said holdingarrangement and for generating a control quantity; and, a vibrationdamping control loop connected to said sensor for receiving said controlquantity to generate a superposition motor current as an actuatingquantity and supplying said actuating quantity to said electric motor tomove said rotational joint via said electric motor so as to counter saidvibration.
 2. The holding arrangement of claim 1, wherein said sensor isone of: an acceleration sensor, a motion sensor or a bending sensor. 3.The holding arrangement of claim 1, said apparatus further comprising abrake assigned to said rotational joint.
 4. The holding arrangement ofclaim 3, said apparatus further comprising a gear assembly for couplingsaid electric motor to said rotational joint.
 5. The holding arrangementof claim 4, said rotational joint defining a rotational axis and saidelectric motor defining a drive axis offset relative to said rotationalaxis.
 6. The holding arrangement of claim 5, wherein said detecting unitincludes an encoder of said electric motor or a position transducer. 7.The holding arrangement of claim 6, said apparatus further comprising anelectronic memory assigned to said control unit; and, said electronicmemory having a curve of current as a function of rotational jointposition or a table of current/rotational joint stored therein.
 8. Theholding arrangement of claim 7, said apparatus further comprising meansfor detecting a time-dependent change of the position of said rotationaljoint.
 9. The holding arrangement of claim 8, wherein said control unitincludes a closed control loop for receiving said time-dependent changeof the position of said rotational joint; and, said control loop outputsa motor current for said electric motor at said rotational joint whichcounters said change of the position of said rotational joint.
 10. Theholding arrangement of claim 9, wherein said control unit superposessaid motor current outputted by said control loop onto said motorcurrent outputted by said control unit to balance said load torqueapplied to said rotational joint.
 11. The holding arrangement of claim1, wherein said rotational joint is a first rotational joint and saidapparatus is a first apparatus and wherein said holding arrangementfurther comprises a second rotational joint and a second apparatus forbalancing a second load torque caused by said instrument and applied tosaid second rotational joint.
 12. The holding arrangement of claim 11,wherein said electric motor of said first apparatus is a first electricmotor and said second apparatus includes a second electric motor; saidcontrol unit includes two control loops; and, each of said apparatusfurther comprises means for detecting a time-dependent change of theposition of the rotational joint corresponding thereto and for supplyingsaid time-dependent change to said control loops; and, said controlloops output two motor currents for corresponding ones of said electricmotors which counter the change of position of said rotational joints.13. The holding arrangement of claim 1, wherein said instrument is takenup with an articulated parallelogram on a carrier arm.
 14. The holdingarrangement of claim 1, wherein said holding arrangement is configuredas a manipulator for moving an instrument.
 15. The holding arrangementof claim 14, wherein a handle is provided for moving said manipulator.16. A method for determining a current control curve for adjusting anequilibrium state in a holding arrangement which includes: at least onerotational joint which is subjected to a load torque by said instrument;and, an apparatus for balancing said load torque applied to saidrotational joint; said apparatus including: an electric motor; adetecting unit for detecting a position of said rotational joint and forproviding a signal representing a position value for said position ofsaid rotational joint; said electric motor being combined with saiddetecting unit; a control unit for receiving said signal and assigning amotor current to said position value; said control unit outputting saidmotor current to said electric motor so as to cause said electric motorto generate a counter torque to balance said load torque applied to saidrotational joint; a device for actively damping vibrations and saiddevice including: a sensor for detecting a vibration of said holdingarrangement and for generating a control quantity; and, a vibrationdamping control loop connected to said sensor for receiving said controlquantity to generate a superposition motor current as an actuatingquantity and supplying said actuating quantity to said electric motor tomove said rotational joint via said electric motor so as to counter saidvibration; the method comprising the steps of: rotating said rotationaljoint about said rotational axis thereof utilizing said electric motor;determining the current demand of said electric motor required to effectthe movement of said rotational joint about said rotational axis;determining the instantaneous position of said rotational joint; and,storing said current demand as a current control curve in an electronicmemory as a function of the position of said rotational joint.
 17. Themethod of claim 16, comprising the further steps of: determining thecurrent demand needed to move said rotational joint in a first directionutilizing said electric motor; moving said rotational joint in saidfirst direction utilizing said electric motor; determining the currentdemand needed to move said rotational joint utilizing said electricmotor in a second direction opposite to said first direction; and,moving said rotational joint utilizing said electric motor in saidsecond direction opposite to said first direction.
 18. The method ofclaim 17, comprising the further steps of: computing a first mean valuefor the current demand needed to move said rotational joint in saidfirst direction; computing a second mean value for the current demandneeded to move said rotational joint in said second direction; and,storing said first and second mean values in said electronic memory independence upon the position of said rotational joint.
 19. The method ofclaim 18, comprising the further step of rotating said rotational jointby a rotational angle of |φ|≦π or |φ|≦π/2 or |φ|≦π/4 utilizing saidelectric motor in order to determine said current control curve.
 20. Amethod for adjusting an equilibrium state in a holding arrangement whichincludes: at least one rotational joint which is subjected to a loadtorque by said instrument; and, an apparatus for balancing said loadtorque applied to said rotational joint; said apparatus including: anelectric motor; a detecting unit for detecting a position of saidrotational joint and for providing a signal representing a positionvalue for said position of said rotational joint; said electric motorbeing combined with said detecting unit; a control unit for receivingsaid signal and assigning a motor current to said position value; saidcontrol unit outputting said motor current to said electric motor so asto cause said electric motor to generate a counter torque to balancesaid load torque applied to said rotational joint; a device for activelydamping vibrations and said device including: a sensor for detecting avibration of said holding arrangement and for generating a controlquantity; and, a vibration damping control loop connected to said sensorfor receiving said control quantity to generate a superposition motorcurrent as an actuating quantity and supplying said actuating quantityto said electric motor to move said rotational joint via said electricmotor so as to counter said vibration; the method comprising the stepsof: determining an instantaneous position of said rotational joint; and,supplying current to said electric motor in correspondence to a currentcontrol curve stored in a memory with said current control curveassigning a current value for torque compensation to said instantaneousposition of said rotational joint.
 21. The method of claim 20, themethod comprising the further steps of: determining an instantaneouschange of the position of said rotational joint; and, outputting acurrent to said electric motor which counters the change of the positionof said rotational joint.
 22. A method for determining a current controlcurve for adjusting a state of equilibrium in a holding arrangementwhich includes: at least one rotational joint which is subjected to aload torque by said instrument; and, an apparatus for balancing saidload torque applied to said rotational joint; said apparatus including:an electric motor; a detecting unit for detecting a position of saidrotational joint and for providing a signal representing a positionvalue for said position of said rotational joint; and, said electricmotor being combined with said detecting unit; a control unit forreceiving said signal and assigning a motor current to said positionvalue; said control unit outputting said motor current to said electricmotor so as to cause said electric motor to generate a counter torque tobalance said load torque applied to said rotational joint; a device foractively damping vibrations and said device including: a sensor fordetecting a vibration of said holding arrangement and for generating acontrol quantity; and, a vibration damping control loop connected tosaid sensor for receiving said control quantity to generate asuperposition motor current as an actuating quantity and supplying saidactuating quantity to said electric motor to move said rotational jointvia said electric motor so as to counter said vibration; said rotationaljoint being a first rotational joint and said apparatus being a firstapparatus; a second rotational joint and a second apparatus forbalancing a second load torque caused by said instrument and applied tosaid second rotational joint; said electric motor of said firstapparatus being a first electric motor and said second apparatusincluding a second electric motor; the method comprising the steps of:detecting the position of said first rotational joint; with the positionof said first rotational joint known, moving said second rotationaljoint about its axis utilizing said second electric motor; determiningthe current demand of said second electric motor needed to move saidsecond rotational joint; determining the instantaneous position of saidsecond rotational joint; storing the current demand in dependence uponthe position of said second rotational joint as a first current controlcurve in an electronic memory; thereafter, and for the known position ofsaid second rotational joint, moving said first rotational joint aboutits axis utilizing said first electric motor; determining the currentdemand of said first electric motor needed for the movement of saidfirst rotational joint; determining the instantaneous position of saidfirst rotational joint; and, storing the determined current demand independence upon the position of said first rotational joint in saidelectronic memory as a second current control curve.
 23. The method ofclaim 22, comprising the further steps of: detecting the position of athird rotational joint; with the respective positions of said first andsecond rotational joints known, moving said third rotational joint aboutits axis utilizing a third electric motor assigned to said thirdrotational joint; determining the current demand of said third electricmotor needed to move said third rotational joint; determining theinstantaneous position of said third rotational joint; and, storing saidcurrent demand of said third rotational joint in dependence upon theposition of said first rotational joint and upon the position of saidsecond rotational joint in an electronic memory as a current controlcurve.
 24. A method for adjusting a state of equilibrium in a holdingarrangement which includes: at least one rotational joint which issubjected to a load torque by said instrument; and, an apparatus forbalancing said load torque applied to said rotational joint; saidapparatus including: an electric motor; a detecting unit for detecting aposition of said rotational joint and for providing a signalrepresenting a position value for said position of said rotationaljoint; and, said electric motor being combined with said detecting unit;a control unit for receiving said signal and assigning a motor currentto said position value; said control unit outputting said motor currentto said electric motor so as to cause said electric motor to generate acounter torque to balance said load torque applied to said rotationaljoint; a device for actively damping vibrations and said deviceincluding: a sensor for detecting a vibration of said holdingarrangement and for generating a control quantity; and, a vibrationdamping control loop connected to said sensor for receiving said controlquantity to generate a superposition motor current as an actuatingquantity and supplying said actuating quantity to said electric motor tomove said rotational joint via said electric motor so as to counter saidvibration; said rotational joint being a first rotational joint and saidapparatus being a first apparatus; a second rotational joint and asecond apparatus for balancing a second load torque caused by saidinstrument and applied to said second rotational joint; said electricmotor of said first apparatus being a first electric motor and saidsecond apparatus including a second electric motor; the methodcomprising the steps of: determining an instantaneous position of saidfirst rotational joint; determining an instantaneous position of saidsecond rotational joint; supplying current to said first electric motorand said second electric motor in correspondence to a current controlcurve stored in a memory; and, wherein said current control curveassigns each of said first and second electric motors a current valuefor torque compensation in correspondence to the respective ones of saidinstantaneous positions.
 25. The method of claim 24, wherein saidholding arrangement further includes a third rotational joint and athird electric motor assigned to said third rotational joint, the methodcomprising the further steps of: determining an instantaneous positionof said third rotational joint; supplying current to said third electricmotor in correspondence to a current control curve stored in a memory;and, wherein said current control curve assigns each of said first,second and third electric motors a current value for torque compensationin correspondence to respective ones of said instantaneous positions.